Microscale devices for high throughput mixing and assaying of small fluid volumes have recently been developed. For example, U.S. Ser. No. 08/761,575 entitled "High Throughput Screening Assay Systems in Microscale Fluidic Devices" by Parce et al. provides pioneering technology related to Microscale Fluidic devices, including electrokinetic devices. The devices are generally suitable for assays relating to the interaction of biological and chemical species, including enzymes and substrates, ligands and ligand binders, receptors and ligands, antibodies and antibody ligands, as well as many other assays. Because the devices provide the ability to mix fluidic reagents and assay mixing results in a single continuous process, and because minute amounts of reagents can be assayed, these microscale devices represent a fundamental advance for laboratory science.
In the electrokinetic microscale devices provided by Parce et al. above, an appropriate fluid is electrokinetically flowed into and through a microchannel microfabricated (e.g., etched, milled, laser-drilled, or otherwise fabricated) in a substrate where the channel has functional groups present on its surfaces. The groups ionize when the surface is contacted with an aqueous solution. For example, where the surface of the channel includes hydroxyl functional groups at the surface, protons can leave the surface of the channel and enter the fluid. Under such conditions, the surface possesses a net negative charge, whereas the fluid will possess an excess of protons, or positive charge, particularly localized near the interface between the channel surface and the fluid. By applying an electric field along the length of the channel, cations will flow toward the negative electrode. Movement of the positively charged species in the fluid pulls the solvent with them. The steady state velocity of this fluid movement is generally given by the equation: ##EQU1## where v is the solvent velocity, .epsilon. is the dielectric constant of the fluid, .xi. is the zeta potential of the surface, E is the electric field strength, and .eta. is the solvent viscosity. The solvent velocity is, therefore, directly proportional to the surface potential. Examples of particularly preferred electroosmotic fluid direction systems include, e.g., those described in International Patent Application No. WO 96/04547 to Ramsey et al., as well as U.S. Ser. No. 08/761,575 by Parce et al. Examples of additional microfluidic fluid manipulation structures relying on pumps, valves, microswitches and the like are described in, e.g., U.S. Pat. Nos. 5,271,724, 5,277,556, 5,171,132, and 5,375,979. See also, Published U.K. Patent Application No. 2 248 891 and Published European Patent Application No. 568 902.
A typical microscale device can have from a few to hundreds of fluidly connected channels chambers and/or wells. Improved methods of making microscale devices which provide for simplified manufacturing, more precise construction and the like are desirable. In addition, the ability to more easily control channel height to width ratios, thereby affecting fluid flow in the channels is also desirable. This invention provides these and many other features.